cxgb3: Correct comparisons and calculations using skb->tail and skb-transport_header

This corrects an regression introduced by "net: Use 16bits for *_headers
fields of struct skbuff" when NET_SKBUFF_DATA_USES_OFFSET is not set. In
that case skb->tail will be a pointer whereas skb->transport_header
will be an offset from head. This is corrected by using wrappers that
ensure that comparisons and calculations are always made using pointers.

Signed-off-by: Simon Horman <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/ethernet/chelsio/cxgb3/sge.c

···
 	p = dui->addr;
 
 	if (skb->tail - skb->transport_header)
-		pci_unmap_single(dui->pdev, *p++,
-				 skb->tail - skb->transport_header,
-				 PCI_DMA_TODEVICE);
+		pci_unmap_single(dui->pdev, *p++, skb_tail_pointer(skb) -
+				 skb_transport_header(skb), PCI_DMA_TODEVICE);
 
 	si = skb_shinfo(skb);
 	for (i = 0; i < si->nr_frags; i++)
···
 	flits = skb_transport_offset(skb) / 8;
 	sgp = ndesc == 1 ? (struct sg_ent *)&d->flit[flits] : sgl;
 	sgl_flits = write_sgl(skb, sgp, skb_transport_header(skb),
-			     skb->tail - skb->transport_header,
-			     addr);
+			     skb_tail_pointer(skb) -
+			     skb_transport_header(skb), addr);
 	if (need_skb_unmap()) {
 		setup_deferred_unmapping(skb, adap->pdev, sgp, sgl_flits);
 		skb->destructor = deferred_unmap_destructor;
···
 
 	flits = skb_transport_offset(skb) / 8;	/* headers */
 	cnt = skb_shinfo(skb)->nr_frags;
-	if (skb->tail != skb->transport_header)
+	if (skb_tail_pointer(skb) != skb_transport_header(skb))
 		cnt++;
 	return flits_to_desc(flits + sgl_len(cnt));
 }